Frost Line Depth in Wisconsin
48 inches under the Uniform Dwelling Code; 40 inches in the southern zone counties
Written by M., a general contractor in Richmond, VA · Last updated · Reviewed against the current Wisconsin Uniform Dwelling Code (SPS 320-325)
How Deep Is the Frost Line in Wisconsin?
The frost line depth for footings in Wisconsin is 48 inches under the Uniform Dwelling Code, Wis. Admin. Code SPS 321.16, which requires footings below the frost penetration level and deems that level 48 in for most of the state. The UDC's frost depth map puts a band of southern and southeastern counties at 40 inches (Kenosha, Racine, Walworth, Rock and neighbors), though Milwaukee and Madison enforce 48. Madison, Green Bay, Eau Claire, La Crosse, Wausau and Superior all use 48 in.
| State | Avg. Depth (in.) | Range (in.) | Major City Examples | Code basis |
|---|---|---|---|---|
| Wisconsin | 65 | 40 - 72 | Milwaukee, Madison, Green Bay | Wisconsin UDC, SPS 321.16: below frost penetration, deemed 48 in (40 in southern zone) |
Returns the code depth for the nearest listed city and the regional note for that part of the state. A ZIP outside Wisconsin still resolves to its own state.
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All 50 states and the national map are on the frost line depth by ZIP code page. Footing width comes from the footing size chart and soil bearing capacity; roof loads from snow load by ZIP code.
What the Wisconsin Code Says
Wisconsin does not use the IRC for houses. One- and two-family dwellings are governed by the Uniform Dwelling Code (UDC), Wis. Admin. Code SPS 320 to 325, written by the Department of Safety and Professional Services and enforced by municipal or county UDC inspectors statewide. SPS 321.16 covers frost protection: footings must extend below the frost penetration level, and the code deems that level to be 48 inches below grade, except in a southern zone shown on the UDC frost depth map where 40 inches is accepted. Municipalities may not adopt a shallower depth.
The 40 in zone covers the counties along the Illinois line and the southern Lake Michigan shore: Kenosha, Racine, Walworth, Rock, Green, Lafayette and Grant, with Waukesha, Milwaukee, Jefferson and Dane historically split or enforcing 48 in as a local policy. In practice most inspectors in the Milwaukee and Madison metros ask for 48 in, and 48 is what you should draw unless the local UDC inspector confirms 40. North of a line through Wausau, measured frost penetration exceeds 60 in in a low-snow winter, and Superior, Rhinelander and Hayward builders commonly go 60 in for exposed piers and unheated buildings.
The UDC allows frost-protected shallow foundations for heated dwellings following ASCE 32 or the department's own tables, and SPS 321.16(2) exempts unheated accessory structures under a size threshold from frost protection, which is how Wisconsin sheds and small detached decks sit on grade. Attached decks, garages and additions all require 48 in. Commercial buildings under the Wisconsin Commercial Building Code (IBC-based) use the engineer's frost depth, usually the same 48. Minnesota to the west uses 42 or 60 in by zone and Michigan 42 statewide; see the national frost line table.
Frost Depth Zones in Wisconsin
The UDC has two zones. Boundary counties and metro enforcement practice are noted so you can confirm with the local UDC inspector.
| Zone or region | Footing depth | Where it applies |
|---|---|---|
| Statewide default (Madison, Green Bay, Appleton, Eau Claire, La Crosse, Wausau, Superior) | 48 in | All counties not in the southern zone; northern counties often build deeper voluntarily |
| Southern zone (Kenosha, Racine, Beloit, Janesville, Monroe, Platteville) | 40 in per SPS 321.16 map | Kenosha, Racine, Walworth, Rock, Green, Lafayette, Grant counties |
| Metro boundary counties (Milwaukee, Waukesha, Dane, Jefferson) | 48 in in practice | Inspectors in Milwaukee and Madison generally require 48 in; confirm before assuming 40 |
| North woods (Rhinelander, Hayward, Ashland, Superior) | 48 in code; 54 to 66 in measured penetration | Oneida, Sawyer, Ashland, Douglas, Vilas counties |
Wisconsin Frost Line Depth by City and County
Values below are the frost depths applied under the UDC in each city. Where a city sits in the southern zone but enforces 48 in, both are noted.
| City | County | Required Footing Depth (in.) | Source |
|---|---|---|---|
| Milwaukee | Milwaukee | 48 | Wisconsin UDC, SPS 321.16; Milwaukee DNS enforces 48 in |
| Madison | Dane | 48 | Wisconsin UDC, SPS 321.16; City of Madison Building Inspection enforces 48 in |
| Green Bay | Brown | 48 | Wisconsin UDC, SPS 321.16 |
| Kenosha | Kenosha | 40 | Wisconsin UDC, SPS 321.16 southern zone (40 in); many builders use 42 to 48 |
| Racine | Racine | 40 | Wisconsin UDC, SPS 321.16 southern zone (40 in) |
| Appleton | Outagamie | 48 | Wisconsin UDC, SPS 321.16 |
| Waukesha | Waukesha | 48 | Wisconsin UDC, SPS 321.16; Waukesha County enforces 48 in |
| Oshkosh | Winnebago | 48 | Wisconsin UDC, SPS 321.16 |
| Eau Claire | Eau Claire | 48 | Wisconsin UDC, SPS 321.16 |
| Janesville | Rock | 40 | Wisconsin UDC, SPS 321.16 southern zone (40 in) |
| La Crosse | La Crosse | 48 | Wisconsin UDC, SPS 321.16 |
| Wausau | Marathon | 48 | Wisconsin UDC, SPS 321.16 |
| Superior | Douglas | 48 | Wisconsin UDC, SPS 321.16 (measured penetration 60 in or more; engineers often specify 60) |
| Sheboygan | Sheboygan | 48 | Wisconsin UDC, SPS 321.16 |
Depth is from finished grade to the bottom of the footing. The 40 in southern zone is a code allowance, not a requirement; a municipality or a builder may use 48 in there and most in the Milwaukee metro do.
Decks, Fences and Water Lines in Wisconsin
- - Deck footings: Any deck attached to the house must bear below the frost line, so piers go to 48 in or deeper. Freestanding decks under the IRC R403.1.4.1 size limit may sit on pier blocks if the local inspector allows it. Size the pier with the deck footing size chart.
- - Fence posts: Code rarely requires frost depth for a fence that carries no structure, but posts set shallower than the frost line heave and lean within a few winters in Wisconsin. Use one third of the post height with a minimum of 24 in, and go to 48 in for gate and corner posts. Wisconsin fence installers typically set 6 ft privacy fence posts 42 to 48 in with 48 in gate posts. Bag counts are in the post hole bags guide.
- - Water lines: IRC P2603.5 puts exterior water service at least 6 in below the frost line and never shallower than 12 in, so the code floor here is 54 in (48 + 6). Many Wisconsin utilities require more; check the water department standard before trenching and call 811 first.
Neighboring states: Minnesota (42 in south, 60 in north), Michigan (42 in), Iowa (42 in), Illinois (42 in Chicago). When a project sits near a state line, the permit follows the jurisdiction where the footing is, not where the contractor is based.
Frequently Asked Questions
What is the frost line depth in Wisconsin?
48 inches under the Uniform Dwelling Code (SPS 321.16), which deems frost penetration 48 in for most of the state. A southern zone of counties along the Illinois line and the southern lakeshore may use 40 in, though Milwaukee and Madison enforce 48.
What is the frost line in Milwaukee?
48 inches. Milwaukee County is at the edge of the UDC southern zone, but the Milwaukee Department of Neighborhood Services and Waukesha County both enforce 48 in on residential footings.
Does Wisconsin use the IRC?
Not for one- and two-family homes. Wisconsin writes its own Uniform Dwelling Code (SPS 320-325), one of the few states that does. The frost provision is SPS 321.16 rather than IRC R403.1.4.1, though the substance is similar.
How deep do deck footings need to be in Wisconsin?
48 in to the bottom of the footing (40 in in the southern zone where accepted) for any deck attached to the house. The UDC exempts small unheated accessory structures from frost protection, so a small freestanding deck may sit on blocks where the inspector allows it.
How deep should fence posts be in Wisconsin?
One third of the post height and at least 24 in for line posts; 48 in for gate and corner posts. Wisconsin's clay-rich glacial soils heave hard; use 6 in of gravel under the post and crown the concrete away from it.
How deep are water lines buried in Wisconsin?
The Wisconsin Plumbing Code (SPS 382) requires water services below frost, and utilities set the practical number: Milwaukee Water Works requires 6 ft (72 in) of cover, Madison Water Utility 6 ft, and northern utilities 7 to 8 ft. Use the utility standard, not the 54 in code floor.
Sources
- Wisconsin UDC, SPS 321.16: below frost penetration, deemed 48 in (40 in southern zone). Retrieved September 27, 2026.
- Wisconsin Legislature. Wis. Admin. Code SPS 321.16, Frost Penetration (Uniform Dwelling Code). Retrieved September 27, 2026.
- Wisconsin Department of Safety and Professional Services. Uniform Dwelling Code program. Retrieved September 27, 2026.
- International Code Council. 2024 International Residential Code, Table R301.2(1), R403.1.4 and R403.1.4.1. codes.iccsafe.org.
- National Weather Service, North Central River Forecast Center. Frost Depth Map. weather.gov.
City values are the frost line or minimum footing depths published or applied by each jurisdiction as of the retrieval date. Codes change on a three-year cycle and local amendments change between cycles; confirm with the building department before you submit a permit.